1. Field of the Invention
The present invention relates generally to submersible downhole pumps. More particularly, the present invention relates to submersible pumps and diffuser assemblies for submersible downhole pumps configured for operation in sandy environments, and methods of pumping a fluid having a substantial sand content.
2. Description of the Related Art
When an oil well is initially completed, the downhole pressure may be sufficient to force the well fluid up the well tubing string to the surface. As the downhole pressure in some wells decreases, some form of artificial lift is required to transport the well fluid to the surface. One form of artificial lift is provided by suspending an Electrical Submersible Pump (ESP) downhole, normally on the tubing string. The ESP is a high speed rotating machine that provides the extra lift necessary for the well fluid to reach the surface. One type of ESP is a centrifugal pump.
Centrifugal pumps have a series of impellers inside of a tubular housing, which are rotated by a drive shaft in order to propel fluids from the radial center of the pump towards the tubular housing enclosing the impellers. The impellers have an inlet or an eye towards the radial center portion around the drive shaft. Spinning the impeller creates centrifugal forces on the fluid in the impeller. The centrifugal forces increase the velocity of the fluid in the impeller as the fluid is propelled towards the tubular housing.
The height that the fluid can travel in a passageway extending vertically from the exit portion of the impeller is the head generated from the impeller. A large amount of head is necessary in order to pump the well fluid to the surface. Either increasing the impeller diameter or increasing the number of impellers can increase the amount of head generated by a pump. The diameter of the impellers is, however, limited by the diameter of the well assembly. As such, the number of impellers is generally increased in order to generate enough head to pump the well fluid to the surface.
During operation, the well fluid enters a stationary diffuser after exiting the impeller. The fluid loses velocity in the diffuser because it is stationary. Decreasing the velocity of the fluid in the diffuser causes the pressure of the fluid to increase. The diffuser also redirects the fluid to the eye or inlet of the next impeller. Each impeller and diffuser combination together form a stage in the pump. The pressure increase from one stage is additive to the amount of head created in the next stage. After enough stages are transversed, the cumulative pressure increase on the well fluid is large enough that the head created in the last impeller pumps the well fluid to the surface.
Each impeller typically mounts directly to the drive shaft, but the diffusers generally slide over the drive shaft and land on the diffuser of the previous stage. A pre-load is applied so that this contact between the diffusers creates a large enough frictional force to prevent the diffusers from spinning with the drive shaft.
The ESPs are often deployed in a sandy, corrosive downhole environment. Various steps have to be taken in the design ESPs to allow for high sand content and the wear that sand particles can cause, to include the use of hardened coatings and abrasion resistant materials. Areas in the ESP that are filled with well fluid that is not in the designed flow path, such as the space between the bottom shroud of the impeller and the diffuser bowl, however, can be problematic. Sand particles can settle between the outer rim of the impeller bottom shroud and the diffuser inner wall where they are trapped in and will roll around until they disintegrate or cut their way through the diffuser wall.
Currently, there are two primary methods used to avoid this destruction. The first is to drill a small hole through the floor of the diffuser's bowl adjacent the inner diameter of the diffuser's bowl (axial wall). This “sand hole” can allow the sand to exit into the lower pressure area of the diffuser entrance. The second is to construct “anti swirl ribs” or “sand dams” in the bowl of the diffuser to keep the sand in place. These ribs look like small “speed bumps” placed radially in the bowl of the diffuser. Both of these methods work to some extent, but both can be over whelmed in some sandy conditions. The sand hole, for example, can erode and enlarge, reducing the efficiency of the ESP. The sand dams, for example, can erode at their outer edge cutting through the dam and eventually the diffuser wall and housing.
Recognized by the inventors therefore is the need for a diffuser (assembly) configured to reintroduce sand trapped between the bottom shroud of the impeller and the diffuser bowl, back into the flow path of the well fluid.